feat: harden storage, schema persistence, fair benches, fix wire crash

Core storage: hash MemTable, WAL group commit, L0 compaction rebuild,
reader-writer lock, and a global StorageGate so HTTP workers and TCP
share the LSM safely under multi-thread access.

Schema: durable CREATE/ALTER/DROP under _schema:tables:* with full LSM
restore on open. Executor types/values/schema split into query/exec/.

Wire protocol: switch default MM to ARC — ORC cycle collector segfaulted
after ~20 async INSERTs. Fair multi-tier benchmarks (SQLite/HTTP/wire/PG)
and honesty docs for mixed-tier comparisons.

benchmarks/compare.nim

@@ -1,4 +1,11 @@
 ## Comparative Benchmarks — BaraDB vs PostgreSQL, Redis, MongoDB
+##
+## ⚠️  SYNTHETIC / PLACEHOLDER: several refTimeSec values are *invented*
+## multipliers, not measured. Do not publish these as real comparisons.
+## Use instead:
+##   nim c -d:release -r benchmarks/bench_all.nim
+##   python3 benchmarks/fair_bench.py
+##   python3 benchmarks/generate_report.py --fair
 import std/times
 import std/random
 import std/strutils